In radio receivers particularly used in digital radio communication, normally, processing such as demodulation is performed on RF (Radio Frequency) signals received at an antenna after converting those signals into signals of low frequency such as intermediate frequency signals or baseband signals. In such case, it is common to employ a method with which: the RF signals are let through a bandpass filter (BPF) to extract signals of a wider band than the band of the received signals; the extracted signals are converted into the intermediate frequency signals or the baseband signals; and further a necessary frequency component is extracted therefrom.
FIG. 9 is an explanatory chart showing the structures of a super heterodyne type radio reception circuit 910 used in an existing radio receiver. The reception circuit 910 first includes: a first bandpass filter 911 (BPF) which passes a particular frequency and a particular bandwidth from RF signals inputted from an antenna 910b; a low-noise amplifier 912 which amplifies output signals from the first bandpass filter 911; a mixer circuit 913 which converts the amplified output signals to intermediate frequency signals; and a local oscillator 914 which supplies local oscillation signals to the mixer circuit 913.
The reception circuit 910 further includes: a second bandpass filter 915 (BPF) which passes a particular frequency and a particular bandwidth further from the intermediate frequency signals outputted from the mixer circuit 913; an A/D converter 916 which performs A/D conversion on the output signals from the second bandpass filter 915; a quadrature demodulation circuit 917 which performs quadrature demodulation on the A/D-converted signals; and a numerical value control oscillator 918 which supplies oscillation signals for operations to the quadrature demodulation circuit 917.
Further, the reception circuit 910 includes: a low-pass filter 919 (LPF) which passes a low frequency side of the output from the quadrature demodulation circuit 917; and a demodulation circuit 920 which demodulates the output signal from the low-pass filter 919 to have an ultimate output signal.
The super heterodyne type reception circuit 910 is in a structure which acquires the intermediate frequency signal from the RF signal by the mixer circuit 913 as described above. Thus, interference occurs when a signal (image signal) of a frequency that is at a symmetric position with respect to a target frequency by sandwiching the oscillation frequency of the local oscillator 914 therebetween is mixed into the input signal of the mixer circuit 913, and it cannot be eliminated at a later stage.
Therefore, it is necessary to eliminate the image signal at the first bandpass filter 911 that is the previous stage of the mixer circuit 913, and the first bandpass filter 911 is required to have a high performance.
FIG. 10 is an explanatory chart showing the structures of a direct conversion type reception circuit 930 that is used in an existing radio receiver. The reception circuit 930 first includes: a bandpass filter 931 (BPF) which passes a particular frequency and a particular bandwidth from RF signals inputted from an antenna; a low-noise amplifier 932 which amplifies output signals from the bandpass filter 931; a quadrature demodulation circuit 933 which converts the amplified output signals to baseband signals; and a local oscillator 934 which supplies local oscillation signals to the quadrature demodulation circuit 933.
The reception circuit 930 further includes: a first low-pass filter 935 (LPF) which passes a low frequency side of the baseband signals outputted from the quadrature demodulation circuit 933; an A/D converter 936 which performs A/D conversion on the output signals from the first low-pass filter 935; a second low-pass filter 937 (LPF) which passes a low frequency side of the output signals from the A/D converter 936; and a demodulation circuit 938 which demodulates the output signal from the second low-pass filter 937 to have an ultimate output signal.
The reception circuit 930 that is the direct conversion type is a structure which converts the signals to the baseband signals not by the super heterodyne type mixer circuit but by the quadrature demodulation circuit 933 as described above. Thus, even if there is an image signal, such signal can be separated by a filter or the like of a later stage. Therefore, compared to the case of the super heterodyne type, not so high performance is required for the bandpass filter 931 of a previous stage.
However, the reception circuit 930 that is the direct conversion type directly converts the RF signal into the baseband signal by the quadrature demodulation circuit 933. Therefore, there are such known issues that DC offset tends to occur, the influence of 1/f noise is likely to be imposed, etc.
FIG. 11 is an explanatory chart showing the structures of a reception circuit 940 that is acquired by partially refining the reception circuit 930 shown in FIG. 10. The reception circuit 940 is a structure acquired by adding a frequency conversion circuit 941 which converts the output signal of an intermediate frequency outputted from the A/D converter 936 to the baseband signal and a numerical value control oscillator 942 which supplies an oscillation signal for operation to the frequency conversion circuit 941 between the A/D converter 936 and the second low-pass filter 937 (LPF) of the reception circuit 930.
The reception circuit 940 performs conversion to the intermediate frequency by using the frequency conversion circuit 941, so that it is easy to eliminate the DC offset, the 1/f noise, and the like.
However, there is such an issue with the cases of any of the reception circuit 910, 930, and 940 of the super heterodyne type and the direct conversion type shown heretofore that the S/N ratio is decreased relatively due to the signal component (spurious) and that the reception sensitivity becomes deteriorated in a case where there are strong signals in the vicinity of the target frequency and such signals cannot be eliminated by the bandpass filter 911 or 931 in the previous stage.
FIG. 12 is an explanatory chart showing a flow of signal conversion in the super heterodyne type reception circuit 910 shown in FIG. 9. FIG. 12A shows the frequency characteristic of the output signal at a stage of output from the first bandpass filter 911, FIG. 12B shows that of a stage of output from the mixer circuit 913, and FIG. 12C shows that of a stage of output from the low-pass filter 917, respectively.
Normally, to be able to deal with a plurality of radio channels, the passband of the first bandpass filter 911 is set to be wider than the bandwidth of the actual receiving signals. Further, the image signals are mixed with the received signals by the mixing processing done by the mixer circuit 913, and it becomes difficult to eliminate those in the latter processing. Thus, the oscillation frequency of the local oscillator 914 is set to a value that is little deviated from the frequency of the received signals so that the image signals can be eliminated completely by the first bandpass filter 911.
In a case where there is a large-power adjacent channel signal in the vicinity of the frequency of the received signals and such adjacent channel signal cannot be eliminated by the first bandpass filter 911, the energy of the adjacent channel signal is also mixing-processed by the mixer circuit 913 simultaneously.
The signals converted into the intermediate frequency signals by the mixing processing go through the A/D converter 916, the quadrature demodulation circuit 916, and the low-pass filter 917 where the adjacent channel signals are eliminated therefrom at last, and are sent to the demodulation circuit 920. That is, when the adjacent channel signals are not eliminated completely by the first bandpass filter 911, the adjacent channel signals are processed along with the received signals, thereby causing deterioration in the S/N ratio.
FIG. 13 is an explanatory chart showing the flow of signal conversion done by the direct conversion type reception circuit 930 or 940 shown in FIG. 10 to FIG. 11. FIG. 13A shows the frequency characteristic of the output signal at a stage of output from the first bandpass filter 931, FIG. 13B shows that of a stage of output from the quadrature demodulation circuit 933, and FIG. 13C shows that of a stage of output from the first low-pass filter 935, respectively.
With this type, the received signal is directly quadrature-demodulated by the quadrature demodulation circuit 933, and converted to a baseband signal having the center of the received signal frequency band as a direct current (DC). In this case, as in the case of the super heterodyne type receiver, the passband of the first bandpass filter 931 is set to be wider than the bandwidth of the actual receiving signal so as to be able to deal with a plurality of radio channels.
Therefore, as in the case of the super heterodyne type, the adjacent channel signals are processed along with the received signals and the S/N ratio is deteriorated also in the direct conversion type when there is a large-power adjacent channel signal in the vicinity of the received signals and the adjacent channel signal is not eliminated completely by the first bandpass filter 931.
As the technical documents related thereto, there are each of following documents. Depicted in Patent Document 1 is a technique which makes it possible to use a plurality of kinds of radio communication services by converting receiving signals to be of an intermediate frequency. Depicted in Patent Document 2 is a reception circuit which controls the band blocking filter characteristics for decreasing the influence of the interference waves for the receiving signals. Depicted in Patent Document 3 is a technique which eliminates a direct current offset by a quadrature demodulator.
Depicted in Patent Document 4 is a reception circuit which uses signals of a plurality of reception bands. Depicted in Patent Document 5 is a reception circuit which recognizes a data transfer rate, and selects the circuit constant accordingly. Depicted in Patent Document 6 is a reception circuit which controls the frequency band and the amplification rate according to the quality of receiving signals.
Depicted in Patent Document 7 is a technique which controls inserted signals of a mixer circuit for eliminating unnecessary image signals. Depicted in Patent Document 8 is a technique which controls detection signals in an FM receiver according to presence of adjacent interference signals.
Patent Document 1: Japanese Patent No. 4456635 (Japanese Patent Application Publication 2008-511260)
Patent Document 2: Japanese Unexamined Patent Publication 2011-193079
Patent Document 3: Japanese Unexamined Patent Publication 2008-079242
Patent Document 4: Japanese Unexamined Patent Publication 2006-319537
Patent Document 5: Japanese Unexamined Patent Publication 2006-262088
Patent Document 6: Japanese Unexamined Patent Publication 2006-109207
Patent Document 7: U.S. Pat. No. 7,272,374
Patent Document 8: U.S. Pat. No. 4,907,293 (Japanese Utility Model Publication Hei 02-032248)
As described in the section of the Related Art, there is such an issue with the cases of any of the reception circuits 910, 930, and 940 of the super heterodyne type and the direct conversion type shown in FIGS. 9 to 11 that the S/N ratio is decreased relatively due to the signal component (spurious) and the reception sensitivity becomes deteriorated in a case where there are strong signals in the vicinity of the target frequency and such signals cannot be eliminated by the bandpass filter 911 or 931 in the previous stage.
In order to prevent such issue, it is necessary to eliminate the adjacent channel signals as much as possible by the bandpass filter 911 or 931 in the previous stage of the reception circuit. That is, the bandpass filter 911 or 931 is required to achieve as high performance as possible.
However, such high-performance bandpass filter normally has a fixed passband, and it is high in price and mounting area thereof is large. In recent digital communication in particular, it is required to be able to deal with a great number of frequency bands (channels) and to be of small in size, low in price, and low in consumption. Therefore, it is not practical to decrease the influence of the adjacent channels by simply using the “high-performance bandpass filter”.
Note here that it is also considered to overcome the above issues through using a variable bandpass filter that can change the passband according to the frequency of the receiving signal as the target. However, the “large-power signals of the adjacent channels” are not necessarily limited to exist with a same frequency at all times. When such signals are generated due to illegal radio or the like, the signals may or may not exist depending on the time zone and the frequency thereof may shift to another frequency.
A case where the frequency of the adjacent channel is changed after setting the variable bandpass filter to a certain passband and a case where the large-power signal of the adjacent channel disappears cannot be dealt with the existing techniques.
Further, to narrow the passband of the variable bandpass filter means to decrease the necessary signal components, so that it is unavoidable to sacrifice the S/N ratio of the target receiving signals to some extent or more. In a case where the large-power signal of the adjacent channel disappears in particular, it is desirable to improve the S/N ratio of the receiving signals by expanding the passband of the variable bandpass filter. However, it is not possible to deal with such case with the existing techniques.
Techniques capable of overcoming the above issues are not depicted in any of Patent Documents 1 to 8. Patent Document 1 discloses a technique which switches the sampling frequency and the like according to the types of the services. Further, Patent Document 5 discloses a technique which switches the circuit constant according to the data transfer rate. However, neither of those techniques is related to the passband of the bandpass filter of the previous stage of the reception circuit.
It is therefore the object of the present invention to provide a radio reception circuit, a radio reception method, and a radio reception program capable of preventing deterioration in the S/N ratio caused by spurious of the high-output adjacent channel signals.